I'm building a power monitor (so I get notifications if the power is lost in one of the inputs). To monitor it safely, I used these 220/12 0.35VA transformers (http://www.hahn-trafo.com/english/pcb-transformers-bv20.php), I have them in a row one next to another. The transformers each are connected like this:
The LED is for indication and the optocoupler is for connecting to Raspberry Pi.
The transformer is rated for 29mA and the actual load is lower (but not by much).
The transformers get quite hot (sticking a temperature probe between two transformers I get 70C). The manufacturers website says that the transformer is rated for "ta 70C/B". Am I still OK or do I need to add a fan?
EDIT: I connected both LEDs in series with total 2K series resistance. The temperature dropped to 58C.
EDIT2: Connected a 390R resistor in series with the diode (the PSU designer software showed that it would reduce the peak current), didn't help. Probably the 1.2W no-load dissipation as specified by the manufacturer adds quite a lot to the temperature. Added a fan to circuate the air (this is in a 1U case). Hopefully it will reduce the tempaerature.
What is a "normal temperature" for a transformer? I know that some bigger transformers get quite hot when working at specified load (no rectifier, just transformer and vacuum tube heater), don't know about the little ones - I do not have a lot of experience with very low power devices such as this.
EDIT3: With a fan the temperature is ~43C on the transformer that is furthest away from the fan. Is that considered good enough? The 58C with no fan looks to me too high, but should be below the 70C spec for the transformer (in case the fan fails).